High in specific strength and specific modulus, fiber reinforced composite materials containing carbon fiber, aramid fiber, etc., as reinforcing fiber have been used widely for manufacturing structural materials for aircraft and automobiles, sporting goods such as tennis rackets, golf shafts, and fishing rods, as well as general industrial applications. A common method widely used for producing these fiber reinforced composite materials is to prepare sheet-like intermediate material called prepreg by impregnating reinforcing fiber with matrix resin and cure a stack of a plurality of such sheets. The use of prepreg have the advantage of producing high performance fiber reinforced composite material easily because the orientation of the reinforcing fiber can be controlled accurately and a high degree of design freedom is ensured for the stack structure. As the matrix resin of such prepreg, thermosetting resin compositions are mainly used from the viewpoint of heat resistance and productivity and in particular, epoxy resin compositions are preferred from the viewpoint of mechanical characteristics such as adhesion to reinforcing fiber.
Fiber reinforced composite materials containing epoxy resin as matrix resin have high heat resistance and good machine physical properties, but epoxy resin is generally lower in elongation percentage and toughness than thermoplastic resin, possibly leading to low impact resistance. Thus, improvements have been called for.
Some attempts, such as adding a tough rubber component or thermoplastic resin, have been made with the aim of producing tougher epoxy resin by curing an epoxy resin composition. However, rubber is significantly lower in elastic modulus and glass transition temperature than cured epoxy resin and therefore, the addition of a rubber component will cause a decrease in elastic modulus and glass transition temperature of matrix resin, making it difficult to maintain a good balance between toughness and rigidity. The proposed methods for blending a thermoplastic component include, for example, adding a copolymer composed of styrene, butadiene, and methyl methacrylate or a block copolymer composed of butadiene and methyl methacrylate in order to provide cured epoxy resin having largely improved toughness (Patent documents 1 and 2). However, these methods have problems such as a decreased heat resistance of cured epoxy resin, deterioration in processability due to increased viscosity of the epoxy resin composition, and deterioration in quality due to generation of voids. Furthermore, Patent document 3 proposes the technique of blending a (meth)acrylic block copolymer with the epoxy resin to produce an alloy. This technique can ensure high toughness, which results from forming a fine phase structure without making the phase separation structure coarse, but further improvements for ensuring higher toughness have been called for.
A disclosed method for providing cured epoxy resin having an improved balance between toughness and rigidity is the use of an epoxy resin composition produced by combining a diglycidyl ether type epoxy compound having a specific number average molecular weight and an epoxy compound that differs from the above epoxy compound in solubility parameter (SP value) in a specific range (Patent document 4). Even with this method, however, the resulting cured epoxy resin will fail to have a sufficiently good balance between toughness and rigidity and furthermore, the epoxy resin composition will be likely to have increased viscosity.
A method available to provide cured epoxy resin with a further improved balance between toughness and rigidity is to use an epoxy resin composition in combination with an epoxy compound with a specific SP value so that a phase separation structure will be formed after curing reaction (Patent document 5). This method can produce cured epoxy resin with high toughness and rigidity by forming a fine phase separation structure after curing and serves to largely improve the performance of matrix resin in conventional fiber reinforced composite materials. However, depending on the reaction conditions, the problem of deterioration in physical properties can occur as a result of a change in the phase separation structure.